1. Field of the Invention
This invention relates to a carrier chrominance signal forming device for forming a carrier chrominance signal by processing an electrical signal obtained from an image sensor through photo-electric conversion.
2. Description of the Related Art
Generally, color television (TV) signals are transmitted in the form of having a carrier chrominance signal multiplexed with a luminance signal. The carrier chrominance signal is formed by modulating color signals obtained from an image pickup system with a color subcarrier of a predetermined frequency.
A modulation device for forming, for example, a carrier chrominance signal for a color TV signal of the NTSC system is arranged as follows. Color signals for colors R (red), G (green) and B (blue) are obtained from an image sensor. Color-difference signals R-Y and B-Y (wherein Y represents a luminance signal) are obtained by carrying out a matrix process on each of the color signals. From the color-difference signals and signals obtained by respectively multiplying the color-difference signals by "-1", a digital data series is formed to include a sequence of (R-Y), -(B-Y), -(R-Y) and (B-Y) according to the period of the color subcarrier specified by the NTSC system. A color-burst signal which has a predetermined phase and level at a specified position (leading edge of each horizontal line) is imparted to the digital data series. The digitial data series with the color-burst signal imparted is D/A converted according to a clock signal of a frequency which is four times as high as the frequency of the color subcarrier. A carrier chrominance signal is formed by restoring the D/A converted signal to an analog signal through a low-pass filter (LPF).
In a case where the color signals obtained from the image sensor are complementary color signals of yellow (Ye), cyan (Cy) and magenta (Mg), it is necessary to form the color-difference signals R-Y and B-Y by forming the color signals R, G and B from the complementary color signals through a computing process and then carrying out the matrix process on these color signals R, G and B. Therefore, a computing process for obtaining a carrier chrominance signal from the complementary color signals requires a circuit of a large scale, which increases the size of the whole apparatus.
In the case of a camera-integrated video tape recorder or the like, a picked-up image signal obtained from an image sensor of an image pickup system is recorded on a recording medium such as a magnetic tape or the like in the following manner. A luminance signal Y and color-difference signals of two kinds R-Y and B-Y are formed from the signal outputted from the image sensor. The luminance signal Y is frequency modulated. The two color-difference signals R-Y and B-Y are subjected to a quadrature two-phase balanced modulation process to form a carrier chrominance signal. The carrier chrominance signal thus formed is subjected to a so-called low-band converting process to make it into a low-band converted carrier chrominance signal. After that, the low-band converted carrier chrominance signal is frequency multiplexed with the frequency modulated luminance signal to obtain a recording video signal for recording on the recording medium.
However, in cases where a carrier chrominance signal is formed by a device of the above-stated kind or where a low-band converted carrier chrominance signal is formed by a camera-integrated video tape recorder, if the signals outputted from an image sensor are, for example, complementary color signals of yellow (Ye), cyan (Cy) and magenta (Mg), it is necessary to form the color signals of R, G and B respectively from these complementary color signals and also to form the color-difference signals R-Y and B-Y by carrying out a matrix process on the R, G and B color signals. In this case, a circuit which performs a computing process for obtaining the carrier chrominance signal or the low-band converted carrier chrominance signal from each of the complementary color signals must be arranged in a large scale, which increases the size of the whole apparatus.